1. Field of the Invention
The present invention relates to a method for manufacturing a silicon single crystal substrate, in particular, to a method for manufacturing a silicon single crystal substrate which is used for growing an epitaxial layer thereon in vapor phase.
2. Description of Related Art
A silicon single crystal substrate (hereinafter, which may be simply called "substrate") is manufactured by slicing, etching and polishing a substrate into which boron (B) for functioning as acceptors, or phosphorus (P), arsenic (As) or antimony (Sb) functioning as donors, were added as dopants. Usage of an epitaxial wafer is available for the measure of latch up or .alpha.-ray soft error, for a MOSLSI device. Such an epitaxial wafer is manufactured by growing a lightly doped epitaxial layer having a dopant concentration of about 1.times.10.sup.15 atoms/cm.sup.3 upon a heavily doped silicon single crystal substrate having a dopant concentration of about 7.times.10.sup.18 atoms/cm.sup.3 in vapor phase.
During a vapor phase epitaxial growth upon a silicon single crystal substrate heated at a high temperature in a hydrogen atmosphere in order to manufacture an epitaxial wafer, chiefly, the rear surface of the substrate is etched by the hydrogen, so that dopants which were heavily added into the substrate are discharged into the vapor phase. The discharged dopants in the vapor phase are trapped within the vapor phase growing epitaxial layer again, that is, the so-called autodoping phenomenon is caused, to make the concentration of dopants in the epitaxial layer non-uniform. In order to suppress the autodoping phenomenon, in a conventional manufacturing method, a protection film of silicon dioxide or silicon nitride is formed on the rear surface of the silicon single crystal substrate to prevent the substrate from being etched by the hydrogen.
However, when forming the protection film on the rear surface of the silicon single crystal substrate by a CVD method (chemical vapor deposition method) using an apparatus 30 shown in FIG. 4, the raw material gas reaches the main surface side of the substrate 10 over the periphery thereof to grow a CVD film 41 thereon, as shown in FIG. 5. Although the grown CVD film 41 on the main surface of the substrate 10 can be removed by a following mirror-polishing step, the grown CVD film 41 on the peripheral side surface thereof remains. In FIG. 4, the apparatus 30 is a continuous processing type of CVD apparatus, the reference numeral 31 denotes a substrate holder, the reference numeral 32 denotes a heater, and the reference numeral 33 denotes a raw material gas feeder. When the CVD film 41 is formed on the side surface adjacent to the main surface of the substrate 10, abnormally grown grain-mass-like polysilicon 45, i.e., the so-called nodules, are formed on the side surface through the CVD film which is porous, during the vapor phase epitaxial growth, as shown in FIG. 6. Such nodules can contribute to generation of particles by separating from the epitaxial wafers during carrying them or the like, or to occurrence of an extraordinary growth what is called a crown 46 which is created in the peripheral portion of the substrate.
For this reason, for example, as described in Japanese Patent Application Publication (Laid-open) No. Tokukai-sho 62-128520, such a protection film 41 has been removed over the whole side surface of the silicon single crystal substrate by using a chemical etching process or a mechanical polishing process, as shown in FIG. 7.
In the case of using a chemical etching process, a cloth impregnated with an etchant is pressed against the peripheral surfaces and peripheral side portion to remove the protection film. However, such a process has the problem that an epitaxial layer having a non-uniform distribution of dopants concentration is formed because a largely waving boundary is formed between the region from which the protection film was removed and the region on which the protection film remains, depending upon the manner of pressing of the cloth, and the amount of autodoping generated during the vapor phase epitaxial growth is locally varied. On the other hand, in the case of using a mechanical grinding process, the grinding is carried out by pressing a grinding stone which is one for grinding against the peripheral portion of the substrate. However, such a process has the problem of remaining a working damage in the ground portion of the substrate firmly, which contributes to generation of stacking faults or slip dislocations when growing of an epitaxial layer. Accordingly, it is indispensable to thereafter carry out removal of remaining working damage, for example, an alkali etching, sufficiently.
Recently, as a silicon wafer used for a power MOSFET for a switching regulator, one in which an epitaxial layer is formed on a heavily As-doped silicon single crystal substrate has been used to lower the ON-resistance thereof.
However, the higher the concentration of arsenic which are doped into the substrate, the larger the amount of autodoping from the substrate becomes, so that autodoping occurred not only from the rear surface of the substrate but also from the side surface thereof have not been negligible.